Extensible lift mechanism

ABSTRACT

The invention relates to a lifting mechanism that includes a base frame and a load supporting frame that are operatively interconnected by a scissor linkage. The load supporting frame is raised and lowered on the base frame by screw drives supporting carriers operatively connected to the load supporting frame. The interconnection between the carrier and the load supporting frame includes linkage means that universally accommodate lateral movement of the load supporting frame relative to the axes of the screws to allow the load supporting frame to be tilted relative to the base frame. The device also incorporates air cushioning supports on the base that allow the entire unit to be floatingly supported on a film of pneumatic fluid for final alignment of the support frame relative to a reference point.

United States Patent 1 1111 3,752,331

Colburn Aug. 14, 1973 EXTENSIBLE LIF! MECHANISM 75 lnvemo h d L. C lb LPrimary Examiner-Gerald M. Forlenza 1 r f z fi Os Assistant ErqminerFrank E. Werner Attorney-Max Dressler, Gerson E. Meyers and [73]Assignee: Fansteel Inc., Chicago, Ill. Ralph R. Roth et a].

[22] Filed: Apr. 19, 1972 57 ABSTRX'C'T [21] Appl' 245,324 The inventionrelates to a lifting mechanism that includes a base frame and a loadsupporting frame that 52 us. c1. 214/1 A, 180/125, 214/512, areoperative/1y interconnected y a scissor linkage- 214 7 1 R, 23 1 7 Theload supporting frame is raised and lowered on the 511 1m. (:1. 13601/02 base frame by screw drives supporting carriers Opera- [58] Field ofSearch 214/1 A, 1 BE, 1 D, tilely connected to the lead supporting frameThe 214/512 701 R 701 p 701 30 5 116, terconnection between the carrierand the load sup- 125 porting frame includes linkage means thatuniversally accommodate lateral movement of the load supporting 5References Cited frame relative to the axes of the screws to allow theUNn-ED STATES PATENTS load supporting frame to be tilted relative to thebase 2 52 695 9 frame. The device also incorporates air cushioning sup-5' Q ports on the base that allow the entire unit to be float- 3'2592557H9 Crgesci 214 D ingly supported on a film of pneumatic fluid for final3:282:359 11/1966 BE x alignment of the support frame relative to areference 3,319,932 5/1967 Szczepanik 214 512 x P 3,474,925 10/1969McCartney et a1 214/512 3,568,804 3 1971 Olsen.... 214/512 X 25 16Drawmg F'gum 620,585 3/1899 Hayes 187/9 Patented Aug. 14, 1973' 7Sheets-Sheet l 7 Sheets-Sheet 2 awn R.

Patented Aug. 14, 1973 Patented Aug. 14, 1973 3,752,331

7 Sheets-Sheet 5 FIG. 8 I60 Patented Aug. 14, 1973 3,752,331

7 Sheets-Sheet 4 FIG. l0

FIG. ll

Patented Aug. 14, 1973 3,752,331

'7 Sheets-Sheet 5 Patented Aug. 14, 1973 3,752,331

7 Sheets-Sheet 7 1 EXTENSIBLE Llr'r MECHANISM BACKGROUND OF THEINVENTION This invention relates generally to material handlingapparatus and more specifically to an improved type of extensible liftmechanism that is capable of raising a load to a predetermined heightabove the ground.

The use of plural framed hoist mechanisms that are interconnected by ascissor linkage and adapted to be raised and lowered relative to eachother have found numerous applications in recent years. Examples of suchunits are shown in U.S. Patents issued to Carder, U.S. Pat. No.3,341,042, and Larson, U.S. Pat. No. 3,246,876. Both such devicesincorporate extensible rams or jacks that are operatively interposedbetween the base frame and part of the structure supported by the baseframe so that extension and retraction of the rams will cause the loadsupporting frame to be raised relative to the base frame.

One major difficulty encountered in the ram operated extensible liftmechanisms of the type under consideration is that varying forces mustbe applied to move a load supporting frame or platform from a fullycollapsed position to a fully extended position, the amount of forcerequired being dependent upon the moment arm for the respective rams.

In order to initiate the upward movement of the load supporting frame,the rams must necessarily be at an angle with respect to the base frameand the load supporting frame. This requires that the collapsedcondition of the lifting mechanism have the platform a considerabledistance above the ground.

A partial solution to the variable force problem of the ram type unitsis found in McCartney et al. U.S. Pat. No. 3,474,925. While thisarrangement allows for the use of a substantially constant lifting forceduring raising and lowering of the support frame relative to the baseframe, one difficulty is that such an arrangement requires that thedrive screws be located substantially at the four corners of therectangular frames to allow the frames to be moved relative to eachother.

A further difficulty is encountered in finally aligning the load orcargo on the load supporting frame relative to its final position thatit is to assume. A unit such as shown in the McCartney et al. patentmust have the load accurately aligned with its final destination inorder to allow the transfer of the load or cargo relative to the cargospace.

A partial solution to this problem is suggested in Shaw U.S. Pat. No.3,370,727.

SUMMARY OF THE INVENTION The present invention is directed to anextensible lift mechanism that is capable of accurately aligning a loadthereon longitudinally, transversely, and vertically relative to anyfixed reference, merely by manipulation of controls forming part of theunit; in which the entire load supporting surface thereof isunobstructed to provide an extremely low profile for the unit in thecollapsed position; and which has a capability of tilting the loadsupporting surface relative to the base for accurate alignment of theload with an adjacent surface.

The extensible lift mechanism of the present invention includes a baseframe and a load supporting frame or platform connected to each other bya scissor linkage and thetwo frames are capable of being moved relativeto each other by a pair of spaced elevating screws located adjacent oneend of the base frame. The lift mechanism includes a reversible drivemeans cooperating with the screws for rotating the screws in oppositedirections.

The screws support first and second carriers for axial movement inresponse to rotation thereof. The two carriers are respectivelyconnected at spaced points on the load supporting frame by first andsecond linkage means that universally accommodate lateral movement ofthe spaced points of the frame relative to the axes of the screws toallow the load supporting frame to be tilted relative to the base frame.

To insure that the load supporting frame can be tilted a substantialamount relative to the base frame, the linkage means accommodatemovement of the spaced points along two perpendicular paths that bothextend perpendicular to the axis of the associated screws.

More specifically, the linkage means include a first link pivoted on thecarrier about a first pivot, a second link pivoted on one of the pointsabout a second pivot on the frame which is parallel to the first pivotand a third link pivoted at opposite ends about spaced parallel pivots,respectively defined on the first and second links. Thelinkage meansallow lateral movement of the frame relative to each of the screwswithout producing any significant lateral forces or strain on thescrews.

To further insure that the load supporting frame can be tilted relativeto the base frame without any distorting forces being applied to eitherframe, the respective arms of the scissor linkage are connected atpivots on the respective frame that include spherical bearings whichallow for lateral or pivotal movement of opposite ends of the armsrelative to the longitudinal dimension of the arms.

According to another aspect of the invention, the drive meansincorporate control means that automatically returns the two frames to asubstantially planar parallel position at extreme ends of travel for thecarri ers on the screws.

According to a further aspect of the invention, the unit incorporates adual mode of support means to support the unit for transportation and toallow free move ment of the unit for final positioning. The first modeof support includes a plurality of caster wheels that are supported onthe load supporting frame and extend below the base frame when the twoframes are in the fully collapsed position. Thus, the unit may beinitially located in close proximity to a final position by movement onits caster wheels, whereupon the two frames can be extended relative toeach other to move the base frame into engagement with the floor orsupporting surface and raise the caster wheels above the floor. Thefinal positioning is accomplished by utilizing a plurality of aircasters or cushions that will produce a supporting film of pneumaticfluid, such as air, so that the entire unit can be freely moved forfinal positioning.

In its preferred embodiment, the extensible lift mechanism isspecifically designed for use in proper positioning of heavy parts foraircraft, such as, for example, landing gears or engines during theassembly or removal thereof relative to the remaining structure of theaircrafi. For this purpose, the unit also incorporates a strut assemblyto allow the load to be longitudinally tilted on the support frame foraccurate alignment of the upper free end of the load with respect tostructure to which it is to be attached.

If desired, the unit may also incorporate a winch to move the load ontothe load supporting frame or platform.

All of the above is incorporated into a very simple unit that can easilybe manufactured with readily replaceable parts that are easilyaccessible for maintenance and replacement.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention and of one embodiment thereof, from the claims and from theaccompanying drawings in which each and every detail shown is fully andcompletely disclosed as a part of this specification in which likenumerals refer to like parts.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWINGS FIG. 1 of the drawingsshows a perspective view of the extensible lift mechanism of the presentinvention;

FIG. 2 is a side elevation view of the mechanism during the loading of alanding gear;

FIG. 3 is a view similar to FIG. 2, showing the landing gear in itssupported position on the lift mechanism;

FIG. 4 is a side elevation view similar to FIG. 3, showing the liftmechanism in a partially extended position;

FIG. 5 is an end view of the lift mechanism showing the load in a tiltedposition on the supporting frame;

FIG. 6 is a fragmentary plan view of the scissor linkage and the frames,with parts thereof broken away;

FIG. 7 is a side elevation of the lift mechanism with parts thereofbroken away;

FIG. 8 is a section view taken generally along line 8-8 of FIG. 7;

FIG. 9 is an end view of the lift mechanism with parts thereof brokenaway;

FIG. 10 is a fragmentary perspective view of the two frames with certainparts being in unassembled condition for clarity;

FIG. 11 is an enlarged fragmentary end view of the one end of the liftmechanism;

FIG. 12 is an exploded view of the various parts forming theinterconnection between the screw carrier and the load supporting frame;

FIG. 13 is a view similar to FIG. 12, showing the various parts inassembled condition;

FIG. 14 shows the side elevation view of the assembled elements shown inFIG. 13;

FIG. 15 is a schematic illustration of the pneumatic circuitincorporated into the mechanism; and

FIG. 16 is an electrical schematic of an exemplary circuit utilized inthe lift mechanism.

DETAILED DESCRIPTION While this invention is susceptible of embodimentin many different forms, there is shown in the drawings and will hereinbe described in detail one embodiment, with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not intended to limit to theembodiment illustrated.

It should also be noted that whilethe unit will be described and hasbeen shown in connection with the positioning of a landing gear, theinvention has applicability in other areas, as will be explained below.

GENERAL DESCRIPTION FIG. 1 of the drawings generally shows theextensible lift mechanism that includes a base or base frame 12 and aload supporting frame or platform 14 that is supported above the base12. The two frames 12 and 14 are interconnected by a scissor linkage,generally designated by the reference 16 (FIG. 4). The extensible liftmechanism further includes drive means, generally designated as 20 forraising and lowering the platform 14 above the base 12. i

The extensible lift mechanism also includes ground engaging elements orcaster wheels 22 (FIG. 2) carried by the platform 14 and extending belowthe base 12 when the two frames are in the totally collapsed condition,for a purpose which will be described later. I

THE PLATFORM DRIVE MEANS.

As was indicated above, the platform drive means is capable of raisingthe platform 14 relative to the base 12 and of tilting the platform 14relative to the base 12 without producing lateral forces on the fixedelements of the drive means. Furthermore, the drive means is of the typewherein all loads are balanced and the forces required forelevating theplatform 14 relative to the base 12 remain constant regardless ofthe'position of the scissor linkage.

The drive means is shown in detail in FIGS. 7 and 9 and includes a pairof columns 30 fixedly secured to the base 12 at opposite comers adjacentone end thereof, which will be referred to as the forward end of themechanism or unit for purposes of orientation only. The columns 30 aresubstantially U-shaped in crosssection and open towards the rear of theunit for a purpose which will be described later.

The columns 30 each support a screw 32 for rotation about a fixed axis.Each screw is supported on a pedestal 34 carried on the base 12 andasuitable bearing 36 fixed to the column at its upper end that hold there spective screws 32 in a substantial vertical position above the base12 wherein the axes of the two screws 32 are parallel to each other. Thepedestal 34 also incorporates a worm gear drive assembly in housing 38that allows the screw 32 to be rotated by a motor 40 having its outputshaft 42 coupled to the input shaft 44 for the worm gear drive.

The motor 40 is of the reversible type and is preferably pneumaticallydriven. For example, the motor may be of the vane type wherein thecentrifugal force holds the vanes tightly against the housing forauniform efficient sea].

A carrier or nut 50 cooperates with each screw for axial movement inresponse to rotation of the screw. Each carrier 50 is connected to onecorner of the load supporting platform 14 through a linkage means 52that will be described later.

According to one aspect of the invention, the drive means consisting ofthe two drive motors 40 preferably capable of being interconnected tooperate as a unit when the platform is to be moved an equal amount onboth sides thereof, or operated independently in opposite directions.

This is accomplished by an electric clutch 54, a clutch adapter 56 and ashaft coupling 58 that interconnect the two shafts 42 of the respectivemotors. When the electric clutch is energized, the two motors areinterconnected so that the output thereof will be simultaneouslyutilized to drive both screws thereby raising the platform an equalamount on opposite sides thereof.

To prevent undue stress on the clutch that interconnects the two motorsduring simultaneous movement of both sides of the platform or loadsupporting frame in the same direction, the motors are preferablysynchronized periodically to prevent one motor from being overworkedwhile the other acting as a vacuum pump. This can be accomplished byproper adjustment of the exhust from the motors in each direction ofdriving, which will be explained later.

THE LINKAGE MEANS The linkage means 52 is capable of allowing thesupporting platform to be tilted in opposite directions from the normalparallel horizontal position above the base without producing anymeaningful lateral stresses on the screw 32. Each linkage means 52between the associated carrier 50 on the screw 32 and the fixed point onthe load supporting frame is identical and one will be described indetail with particular reference to FIGS. 12, 13 and 14.

The linkage means 52 includes first and second depending lugs 100 thatare respectively fixedly secured to opposed surfaces of the carrier 50by screws 102 and pins 104. The lower ends of the lugs each have anopening 106 defined therein with the openings 106 being aligned when thelugs 100 are in the assembled condition on the carrier or nut 50. Theseopenings 106 thus define a first pivot axis that extends perpendicularto the axis of the screw 32.

The linkage means 52 further includes a second pair of lugs 1 that arefixedly secured to a connector 112 through bolts 114 and nuts 116 aswell as washers 118. Lugs 110 each have an opening 120 therein thatdefine points fixed relative to the support frame which is secured tothe connector 112 by a bracket 121 (FIG. 7).

The linkage means 52 further includes a first link 122 that is pivotedon the carrier about a first pivot defined by openings 106. For thispurpose, the link 102 has first and second circular projecting pins 124extending from the diametrically opposed points on the periphery of thelink. The linkage means also includes a second link 126 that hasoutwardly projecting pins 128 received in openings 120 that define asecond pivot which extends parallel to the first pivot and both pivotsextend perpendicular to the axis of the screw 32. An inspection of FIG.12 shows that the links 122 and 126 are circular rings, identical inconstruction.

The linkage means 52 also includes means in the form of a third link 130interconnecting the first and second links 122 and 126. The third linkincludes the first and second link segments 130a and 130b that arepivoted at opposite ends about spaced parallel pivots respectivelydefined on the first and second links. The pivot on the first link isproduced by the outwardly extending circular projecting pins 132 on link122 with 1 identical projecting pins 134 on the second link 126.

The respective projections 132 and 134 are received in openings 136 and138 respectively defined on opposite ends of both link segments 130a and13011. The link segments are retained on the projections by screws 140and washers 142.

The third link 130 is therefor pivoted at opposite edns about third andfourth spaced parallel pivots that are respectively defined on the firstand second links 122, 126 and these pivots extend perpendicular to thefirst and second pivots defined by projections 124 and 128 and alsoextend perpendicular to the fixed axis of the screw 32.

With the parts interconnected in the manner shown in FIG. 13, anylateral movement of the platform 14, which is connected to the member 112, in the direction of arrows 144 will cause the linkage means oruniversal connection 52 to pivot about the spaced pivots 124 and 128 toprevent any meaningful lateral forces from being developed on the screw32. Likewise, any lateral movement in the direction of arrows 145 willcause the second link 126 to be moved relative to the first link 122 andsuch relative movement will cause the link to pivot about the respectivepivots on opposite ends thereof. The movement along the arrows wouldcorrespond to forward and rearward movement of the platform 14 relativeto the base frame while the movement along the arrows 144 wouldcorrespond to movement of the platform from side to side.

In order to accommodate the pivotal movement of the links relative tothe screws 32, the two links 122 and 126 as well as the connector 112all have enlarged openings 146 through which the screw extends to allowunobstructed lateral movement of the links and the members relative tothe screw 32.

THE SCISSOR LINKAGE As was indicated above, the scissor linkage 16 (FIG.4) causes the rear end of the platform or load supporting frame to movein response to movement of the forward end of the platform by rotationof the respective screws 32. In addition, the scissor linkage isconnected to the respective frames to accommodate the tilting movementof the load supporting frame with respect to the base frame without anybinding forces being applied to the structure.

Since the scissor linkage 16 includes first and second pairs of armsthat are interconnected to each other and connected to the respectiveframes in an identical manner on opposite sides of the unit, only onepair of arms will be described in detail with particular reference toFIGS. 6, 7 and 8.

The scissor linkage 16 includes first and second arms and 162 that arepivotally interconnected by a pin 164. The first arm 160 consists offirst and second transversely spaced portions 160a and 16012interconnected at opposite ends by plates 164. The spacing between theportions 160a and 160b is such that the second arm 162 may be locatedwithin the first arm, as will be described later.

The first arm 160 is connected at one end on the base frame about afixed pivot by a lug 168 which extends between a pair of spaced lugs 172(only one being shown) and a pivot pin extends through openings in thelugs.

The second arm 162 is also pivoted about a fixed pivot on the loadsupporting frame or platform. The fixed pivot again includes a pin 175extending between spaced lugs 176. Both pivotal connections between thepins and the respective arms includes a spherical bearing (not shown) toaccommodate pivotal movement of the arms transversely of the unit, aswill be explained later.

The opposite ends of both arms 160, 162 are respectively connected tothe base 12 and platform 14 by movable pivots, that allow longitudinalmovement of the arm ends in response to vertical movement of the frames.As shown in FIG. 8, the base 12 and the platform 14 each have a pair oftrack elements 180 each having a vertical inner surface with a recess182 defined therein. A pivot pin 184 extends into the transverselyaligned recesses 182 and the arms 160,. 162 are pivoted on the pin 184.The upper portion of FIG. 8 shows the construction of the pin 184 incross-sectional detail. The pin 184 includes a sleeve 186 having aspherical bearing 188 supported thereon and retained between oppositeends by a pair of rollers 190. Each roller 190 has a reduced portion 192received in the elongated recess 182 and the assembly is held togetherby a bolt 194 and a nut 196.

The end of the first arm 160 has an opening in which a cooperatingbearing member 197 is secured and the bearing member is retained in theopening by a pair of bearing plates 198 secured to opposite sidesthereof. Thus, the arm 160 can universally pivot about the pin 184 toaccommodate lateral tilting of the respective frames relative to eachother as well as pivoting about the axis of pin 184, as will beexplained later.

In order to have the two arms completely collapsible and have the secondarm located entirely within the first arm, the first arm has adownwardly extending portion on the first end which is connected to thepivot pin 170 while the second or opposite end has an upwardly extendingportion connected to the pin 184. Likewise, the second arm has upwardlyand downwardly extending portions on opposite ends which are reversed.With this arrangement, the two arms can be completely collapsed to thesolid line position shown in FIG. 7 so that the unit will have anextremely low profile in the completely collapsed position.

THE UNIT SUPPORT MEANS As was indicated above, the extensible liftmechanism or unit incorporates a first mode of support for movement fromone site to another and a second mode of support for final positioningof the unit at its ultimate destination.

The first mode of support has been generally discussed above andincludes the four caster wheels 22 that are supported on the loadsupporting platform 14 for movement therewith. As more clearlyillustrated in FIGS. 7 and 11, the caster wheels 22 are supported on theupper frame 14 by a carriage 202 that extends through an opening 204 inthe base 12 adjacent each corner thereof. The upper end of the carriage202 is supported on the cross plate 206 that extends between frameelements 208 that define a rectangular support for the carriage (seeFIG. 7). The casters 22, as indicated above,are raised and lowered withthe platform 14 so that initial relative movement between the platformand the base will cause the base to move downwardly into engagement withthe ground. The first mode of support and transportation also includes atow bar 210 (FIGS. 6 and 7) that is connected to the base at the forwardend thereof.

After the device to be transported has been positioned on the platform14, the platform 14 and base 12 are completely collapsed so that thebase raised above the ground a sufficient distance to permit the casters22 to make contact to allow the unit with the load or cargo thereon tobe moved to its ultimate destination. Of course, it will be appreciated,if desired, the entire unit may be of the self-propelled type byproviding suitable propulsion for the caster wheels in the manner thatis known in the art.

The second mode of support comes into operation at the final destinationfor the load or cargo to accurately position the load with respect to afixed reference. A second mode of support is more clearly shown in FIGS.7, 10 and 11. A second-support means consists of four resilientinflatable members or air casters 220 that are supported at the fourcomers of the base frame. Since all four casters are identical, only onehas been shown in detail in FIG. 10 and particular reference will bemade thereto.

The inflatable means 220 consists of a rigid plate 222 having aresilient elastomeric member 224 secured to the lower surface thereof.The elastomeric member 224 is a flat disc of high strength elastomersreinforced with high strength fabrics that takes the form of a donut,when inflated, having a recess 226 at the center thereof. A deflector227 is positioned in the recess 226.

The base frame 12 has a pair of spaced guide means i 228 having inwardlydirected flanges so that the inflatable means, more particularly theplate 222 may be slidably inserted between the guides and held in fixedrelation by suitable sealing means. The base frame also has an inletopening 229 defined therein that cooperates with an aligned opening (notshown) in the plate 222 when the plate is in the assembled position.

Thus, when the pneumatic fluid, such as air, issupplied through theinlet opening 229, the air initially inflates the flat disc to a donutconfiguration to lift the entire load a small increment above the groundor floor. After initial inflation, the recess 226 defined by the ring224 serves as a large piston into which air is supplied through suitablecommunication means or openings along the inner periphery of the ring224 that accommodate flow of fluid from the inflatable member. The airis then deflected downwardly by the deflector 227 and escapes through asmall opening that will be created between the lower surface of thedonut shaped ring and the supporting surface, such as a floor in thebuilding. This flow of air will produce an extremely thin air film thatcompletely frees the base frame from the floor and the resiliency of thedonut shaped ring automatically contours to slight irregularities in thefloor to provide a constant air gap and uniform air flow between thefloor and the lower edge of the ring.

The inflatable means 220 is a unit commercially available from Aero-GoInc. 5820 Corson St Seattle, Wash. that includes a seal surrounding theair entrance which is in the form of a double adhesive backed spongerubber tape having a protective cover thereon. In installing theinflatable means or air caster, the protective cover is removed from thetape and the entire unit is slid along the guides 228 until it engagesin a stop (not shown). Thereafter, pressure is exerted on the but tom ofthe air caster to engage a seal with the lower surface of the baseframe.

The second mode of support also includes brake means for temporarilyholding the floating unit in a fixed position on the floor while thethin film of air supports the unit. This means is shown in FIG. 11 andincludes a ram with a cylinder 230 having a piston rod assembly 232reciprocable therein. The outer end of the piston rod assembly has afoot 234 secured thereto. The ram is of the single acting type whereinthe piston rod assembly is normally held in retracted position by aspring 236 (FIG. 15). By supplying fluid to the head end of the cylinder230, the foot 234 is moved to engage the floor and acts as a brake tohold the unit in a fixed position.

ADJUSTABLE HOLDING MEANS As was indicated above, the description anddrawing disclose the present invention in a specific environment for usein accurately positioning aircraft components with respect to a fixedreference for installation. For example, in extremely large aircraft,such as the B-747, the components are extremely large, heavy, and bulkyand must be accurately positioned for attachment to the body structureof the aircraft. In the specific illustration, the load or cargo on thelift mechanism is illustrated as a wing gear L that must be positionedwith respect to a wing for attachment thereto. This many times requiresmovement of portions of the wing gear for accurate alignment withopenings in the wing.

Thus, the extensible lift mechanism further includes a strut assembly250 pivoted on a support bracket 252 extending above the platform orload supporting frame 14 at a location substantially between the twocolumns 30. With particular reference to FIG. 7, the strut assembly 250consists of a base 254 that is pivoted about pins 256 on the supportbracket 252. A rod 258 extends through an opening in the base and isreciprocable relative to the base by an air motor 260 and a worm geardrive between the rod and motor. A bellows 262 surounds the screw driveportion of the rod when it is extended. Pneumatic fluid, such as air,under pressure drives the motor in either direction to move the rod 258relative to the base 254.

CARGO LOADING ATTACHMENT For extremely heavy cargo, such as a landinggear structure, it is also desirable to provide a loading attachment forinitially placing the cargo on the platform 14. The loading attachmentis shown in FIGS. 2 and 7 and includes a winch 270 consisting of an airmotor 272 that drives a cable reel 274 supported on the platform 14 witha cable 276 extending therefrom. The rear end of the platform has a pairof ramps or runners 280 pivotally secured thereto on pins 282 so thatthe rear ends of the ramps 280 may be pivoted toward the ground. The airmotor driven winch can then be attached to the landing gear and the airmotor actuated to pull the landing gear onto the supporting surface. Thelanding gear can then be maintained on the platform surface by suitablechocks 290 (FIG. 4).

PNEUMATIC CIRCUIT The pneumatic circuit for supplying fluid underpressure to the respective motors 40, 260 and 270, as well as the brakeand the air casters 220, is schematically illustrated in FIG. 15. Thepneumatic circuit includes a source of pneumatic fluid under pressure300 connected to a conduit 302 with an air filter 304 and a lubricator306 located therein. The pneumatic or air supply is delivered to therespective motors 40, 260 and 270 through four-way double solenoidspring center valves 310, 312, 314 and 316. Thus, the position of therespective valves, which is controlled by the solenoids, determines thedirection of rotation of each of the motors.

The air supply conduit 302 is connected by a conduit 320 to a two way,single solenoid, spring return, normally closed valve 322 which suppliesfluid to a pair of branch conduits 324. Each branch conduit 324 has an10 air regulator 326 incorporated therein and leads to two air casters220. A further adjustment valve 328 is located in each of the branchconduits between the two casters.

With the pneumatic circuit as shown, the pressure regulator 326 can beadjusted to control the supply of fluid to respective pairs of aircasters located on opposite sides of the lift mechanism to compensatefor an imbalance of the pladorm that may result from the load beinglaterally offset on the platform. Furthermore, the control of thepressure of fluid between the fore and aft air casters is controlled bythe valves 328. Preferably, the two air casters located adjacent theregulators would be located in the aft end or rear end of the liftmechanism where the majority of the load would normally be centered.

The supply of air to the pneumatic brakes or rams is directed through aconduit 330 having a two-way, single solenoid, spring return, normallyclosed valve 332 and a needle valve 334 located therein.

ELECTRICAL CIRCUIT The electrical circuit for supplying energy to thevarious solenoids on the valves in FIG. 15 as well as the electricclutch 54 is shown in FIG. 16. The electric circuit includes aconnection 400 for a power supply to the lift mechanism. A master switch402 is incorporated in the two lines 404 and 406 leading from the sourceto the transformer 408 with suitable fuses 410 incorporated therein. Thetransformer produces a 12 volt output supply with one line of the outputconnected to each of the 8 solenoids through a line 414. The output ofthe transformer 414 is also connected through a line 416 and a normallyopen switch 418 to the input contacts of a tilt DPDT spring centeredswitch 420 and to the input contacts of a up-down 3 PDT, spring centeredtoggle switch 422.

The first contact 420-1 of the switch 420 is connected by a line 424through a limit switch 426 and one of a pair of contacts of the limitswitch 428 to solenoid 430 on one end of the valve 310 that controls theflow of fluid to the right hand motor 40. The second contact 420 2 ofthe switch 420 is connected through a line 432, limit switch 434 and onecontact of limit switch 436 to the second solenoid 438 on valve 310.

The third contact 420 -3 of switch 420 is connected through line 440,limit switch 442, one contact of limit switch 444 to solenoid 446 whichis on one end of the valve 312 leading to the left hand motor 40. Afourth contact 420-4 of switch 420 is connected through line 450, limitswitch 452, one contact of limit switch 454 to solenoid 456 on theopposite end valve 312.

The first contact 422-1 of switch 422 is connected through line'460 tosolenoid 430'through the first contact of limit switch 428. The secondcontact 422-2 is connected through line 462 and the first contact oflimit switch 436 to solenoid 438. The third contact 422-3 of switch 422is connected through line 464, and the first contact of limit switch 454to solenoid 456. The fourth contact 422-4 of switch 422 is connectedthrough line 466 and the first contact of limit switch 444 to solenoid446. The remaining two contacts 422-5, 422-6 of the switch 422 will bedescribed in connection with the circuit to the clutch 54.

The two switches 420 and 422 control the direction of the respectivescrew drive 32. If for example, the platform is to be raisedsimultaneously on both sides,

switch 422 is moved to the uppermost position energizing solenoids 430and 456 to drive the screws 32 in the direction to move the carriers 50upwardly while movement of the switch 432 to its lowermost position willenergize solenoids 438 and 446 to lower the platform with respect to thebase.

A separate electric energy supply for the clutch 54 is delivered fromthe bridge rectifier 470 to one side of the clutch 54 through a line 472having a toggle switch 474 therein. The second output contact ofrectifier 470 is connected to the third input contact of the switch 422through a line 480 having a fuse 482.

The fifth contact 422- cooperating with the third input contact ofswitch 422 is connected through a line 482, the other contact of limitswitch 454, and the other contact of limit switch 428, to the clutch 54.The sixth contact 422-6 of switch 422 is connected through line 490, theother contact of limit switch 436 and the other contact of limit switch444 to the clutch through line 486.

The respective limit switches 428, 436, 444 and 454 are located atopposite ends of the path of travel of the respective carriers and arenormally closed so that the clutch may be controlled through the switch474. However, assuming that the right hand side of the platform ishigher than the left hand side, and the switch 422 is actuated toenergize solenoids 430 and 456, when the right hand carrier reaches theupper extreme position or limit of travel, limit switch 428 would beopened to interrupt the circuit to solenoid 430 thereby closing thevalve 310 to the right hand motor while the left hand motor wouldcontinue to operate until the left hand carrier reached its upperextreme limit of travel, whereupon limit switch 444 would be actuated tointerrupt the circuit to solenoid 446, The same condition would occurwith respect to downward travel by appropriate actuation of switches 436and 444. It will also be appreciated that the respective limit switches426, 434, 442 and 452 would be appropriately positioned with re spect tothe platform to define the maximum degree of tilt in either directionfor the platform and, when this maximum degree of tilt is reached, theappropriate mercury switch would interrupt the circuits to the varioussolenoids.

The control to the strut tilt assembly motors 260 include a SPDT, springcentered toggle switch 500 that has its input contact connected to oneoutput lead of the transformer 408 through a line 502. The first outputcontact 500-1 of switch 500 is connected through line 502 and limitswitch 504 to solenoid 506 while the second output contact 500-2 isconnected through line 508 and limit switch 510 to solenoid 512. Thesolenoids 506 and 512 are respectively located on the opposite ends ofthe valve 314 which controls the flow of fluid to the strut motor 260.Actuation of the toggle switch 500 in the appropriate direction willenergize through the solenoid 506 or 512 to extend or retract the strutor rod 258. The respective limit switches 504 and 510 would define theextreme positions of travel for the rod or strut.

The circuit to solenoid 520, forming part of valve 332,includes a switch522 having both output contacts connected through line 524 to thesolenoid 520 so that actuation of the switch in either direction willenergize the solenoid and extend the piston rod in the brake.

The solenoid 530 forming part of the valve 322 is connected totransformer output through a line 532 and a switch 534.

The operation of the entire circuit will be described later inconnection with a loading and unloading operatron.

ADJUSTMENT OF DRIVE MOTOR 40 As was indicated above, it is preferable toadjust the speed of the two drive motors 40 to assure that both motorsare synchronized when the clutch is actuated and the screws are bothbeing rotated in the same direction to prevent undue stress on theclutch as well as the motor. This is accomplished by a plurality ofadjustable valves 350 cooperating with the exhaust port of therespective valves 310 and 312. This adjustment of the speed of therespective motors should be accomplished each time the air supplythrough the unit is changed to prevent the above mentioned undesirableresults.

The same control may be incorporated into the exhaust ports of the strutcontrol valve 314 to control the speed of the motor 260.

OPERATION The operation of the unit will be described in connection withthe loading, transporting and final positioning of a landing gear.Initially, the rear end of the lift mechanism or the unit is alignedwith the landing gear L (FIG. 2) and the ramps 280 are lowered. T hewinch motor is then actuated by the manual actuation of valve 316 toallow the cable 276 to be unwound and attached to the forward end of thelanding gear I... The valve 316 is then manually actuated in theopposite direction to pull the landing gear onto the platform 14 to theposition shown in FIG. 3. At this time, the checks 290 may be positionedadjacent the forward and rearward wheels of the landing gear and theramps 280 raised for transportation.

The rod or strut 258 is appropriately extended and attached at asuitable location to the upper portion of the landing gear so that theentire unit is substantially fixed relative to the lift mechanism. Apowered vehicle, such as a tractor, can then be attached to the drawbarand the landing gear transported to its ultimate site of installationutilizing the caster wheel support. After the lift mechanism has beenproperly positioned with respect to the point of installation on theaircraft, the updown switch 422 is moved to the upward direction toenergize solenoids 430 and 456, and clutch 54 simultaneously casuingboth sides of the platform to be raised from the position shown in FIG.3 to be that shown in FIG. 4. During the initial movement of the twoplatforms relative to each other, the base frame 12 will be lowered intoengagement with the adjacent floor F and subsequently the platform orupper support frame 14 will be elevated above the base frame. When theupper end of the landing gear is in general alignment with the attachingpoint of the aircraft, switch 422 is released to interrupt the circuitto the respective solenoids.

At this time it may become necessary to tilt the landing gear either'inthe forward and rearward direction or ally shown in FIG. 4. The tiltingof the landing gear transversely is accomplished by actuating the switch422 in the appropriate direction to energize the respective solenoidscooperating with the motors 40. When the final angular position andlongitudinal position has been reached, it may also become necessary toreposition the entire structure relative to the body of the aircraft.This is accomplished by energizing solenoid 530 to supply pneumaticfluid to the respective air casters 222 that will result in floatinglysupporting the entire unit and cargo on a thin film of fluid to allowthe operator to manually move the unit with a very small amount of forceto its final position relative to the point of installation.

It should be noted that during the tilting movement, the platform willgenerally pivot about an axis extending centrally of the elongatedplatform since'both motors are rotating the screws at the same speed andin opposite directions This tilting movement will result in having thepoints of connections of the platform move laterally relative to thecarriers 50. This lateral movement will cause a pivoting of theappropriate link in the linkage means 52 and prevent any lateral forcesfrom being applied to the screws 32. The lateral movement will alsocause a small amount of pivotal movement of the respective ends of thearms about the spherical bearing supports.

After the platform has been tilted the desired amount on the base, itmay become necessary to raise or lower the platform while it is in thetilted position. This may be accomplished by actuating the switch in theappropriate direction to drive both screws in the same direction.Because the motors operate at the same speed, the angle of tilt of theplatform will remain the same. Also, the angular position of the scissorlinks between the platform and base, when the platform is tilted, willadd stability to the platform.

SUMMARY It will be seen that the present invention provides extremelyrugged structure that can be utilized for transporting various types ofloads or cargo a substantial distance and allow a universal movement ofthe entire unit for final positioning relative to a fixed point. Inaddition, a platform can readily be tilted to accommodate the angle areceiving unit, such as an aircraft, may be located.

To reiterate, while the lift mechanism has been shown as described inconnection with the transportation and final positioning of extremelyheavy units that require accurate positioning, numerous other uses areavailable for a unit of this type. For this example, the expansiblemechanism with the tilt capability could be readily utilized as a cargoloader for other material handling equipment.

I claim:

1. In an extensible lift mechanism having a base riersand spaced pointson said load supporting frame, said linkage means universallyaccommodating lateral movement of said spaced points relative to saidaxes in response to differential rotation of said screws to allowtilting of said load supporting frame relative to said axes.

2. An extensible lift mechanism as defined in claim 1, in which eachlinkage means includes links accommodating lateral movement in twointersecting paths perpendicular to said axes of said screws. i

3. An extensible lift mechanism as defined in claim 1, in which eachlinkage means includes a first link means pivoted on said carrier abouta first pivot; a second link means pivoted on one of said points on asecond pivot parallel to said first pivot, said first and second pivotsextending perpendicular to said fixed axes; and means interconnectingsaid first and second link means.

4. An extensible lift mechanism as defined in claim 3, in which saidmeans interconnecting said first and second link means includes a thirdlink means pivoted at opposite ends about spaced parallel third andfourth pivots respectively on said first and second link means, saidthird and fourth pivots extending perpendicular to said first and secondpivots and said fixed axes.

5. An extensible lift mechanism as defined in claim 1, in which saidload supporting frame includes floor engaging elements, said floorengaging elements being dimensioned to raise said base frame above afloor upon movement of said carriers to an extreme position adjacentsaid base frame.

6. An extensible lift mechanism as defined in claim 5, further includinginflatable means defining recesses below said base frame; and means forsupplying pneumatic fluid to initially inflate said inflatable means andflow into and from recesses between said inflatable means and the floorto produce a film of fluid to floatingly support said base frame on thefloor.

7. An extensible lift mechanism as defined in claim 6, including brakemeans on said base frame for selectively holding said base frame in anadjusted position while said base frame is floatingly supported on thefloor. 7

8. An extensible lift mechanism as defined in claim 1, in which saidframes are polygonal and said screws are located at adjacent corners ofone end of the said base frame.

9. An extensible lift mechanism as defined in claim 8, in which saidscissor linkage includes first and second pairs of arms pivotallyinterconnected intermediately opposite ends, one end of each first armpivoted, on one end of said base frame adjacent said screws and one endof each second arm pivoted about a fixed pivot on said load supportingframe adjacent said screws; guide track means on said base frame andsaid load supporting frame, movable pivots for opposite ends of saidarms guided in said track means, and in which all of said pivots forsaid arms accommodate lateral movement of said frames relative to eachother.

10. An extensible lift mechanism as defined in claim 1, furtherincluding control means cooperating with said reversible drive means forselectively rotating said screws in opposite directions to tilt saidload supporting frame relative to said base frame.

11. An extensible lift mechanism as defined in claim 10, in which saidcontrol means includes means for raising and lowering said loadsupporting frame relative to said base frame while maintaining a presetangle of tilt of said load supporting frame relative to said base frame.

12. An extensible lift mechanism as defined in claim 1 l, in which saidcontrol means includes means for automatically leveling said loadsupporting frame at opposite extreme limits of travel of said carrierson said screws. I

13. An extensible lift frame as defined in claim 1, further includingcontrol means for cooperating with said drive means for rotating saidscrews synchronously or independently.

14. An extensible lift frame as defined in claim 1, in which saidcontrol means includes means for simultaneously rotating said screws inthe same direction or in opposite directions.

15. An extensible lift mechanism as defined in claim 1, in which saidscissor linkage includes first and second pairs of arms pivotallyinterconnected intermediate opposite ends, each arm having one endconnected by a fixed pivot to one of said frames and an opposite endconnected by a movable pivot to the other of said frames and in whichsaid connections of said arms include spherical bearings accommodatingtilting of said load supporting frame relative to said base frame.

16. An extensible lift mechanism as defined in claim 1, in which saiddrive means includes a pneumatic motor for each of said screws.

17. An extensible lift mechanism as defined in claim 16, furtherincluding releasable coupling means for interconnecting said motors tosynchronously drive both screws with both motors.

18. In an extensible lift mechanism having a horizontal base frame and aload supporting frame with scissor arms interposed between said frames;first and second screws extending above said base frame at spacedlocations and supported for rotation about substantially vertical axes;drive means for rotating said screws in opposite directions on said baseframe; carrier means cooperating with each screw; and linkage meansconnecting said carrier means to said load supporting frame at spacedlocations, said linkage means universally accommodating lateral movementof said load supporting frame relative to said vertical axes uponselective rotation of said screws for tilting said load supporting framerelative to said vertical axes while minimizing lateral forces on saidscrews.

19. An extensible lift mechanism as defined in claim 18, in which saidlinkage means includes a plurality of links pivotally interconnected andpivoted to said carrier means and supporting frame to accommodatelateral movement of said supporting frame in two intersecting pathsperpendicular to said axes.

20. in an extensible lift mechanism including a base frame and a loadsupporting framesupported above said frame by scissor arms; a carrier;drive means for moving said carrier in opposite directions along a fixedaxis extending above said base frame; and connection means between saidcarrier and supporting frame, said connection means including linkagemeans having opposite ends respectively pivoted about spaced parallelaxes respectively defined on said carrier and said supporting frame,said spaced parallel axes extending perpendicular to said fixed axis andsaid linkage means universally accommodating lateral movement of saidsupporting frame relative to said fixed axis.

21. An extensible lift mechanism as defined in claim 20, in which saidlinkage means includes a plurality of link means pivctallyinterconnected and pivoted about said spaced parallel axes toaccommodate lateral movement of said supporting frame in pluraldirections perpendicular to said fixed axis.

22. An extensible lift mechanism as defined in claim 21, in which saidlinkage means includes a first link means pivoted on said carrier, asecond link means pivoted on said supporting frame and third link meanspivoted at opposite ends on said first and second link meansrespectively.

23. In an extensible lift mechanism including a base and a platform; ascissor linkage interposed between said base and said platform; meansfor extending and retracting said platform above said base betweencollapsed and extended positions; a plurality of spaced wheels securedto said platform, said wheels extending below said base to engage asupport surface and support said lift mechanism when said base andplatform are in a collapsed position; a plurality of inflatable membersextending from a lower surface of said base; means for selectivelysupplying pneumatic fluid under pressure to said inflatable members; andmeans accommodating flow of fluid from said members to produce asupporting film of fluid between the inflatable members and said supportsurface to support said lift mechanism on said supporting film of fluidwhen said base and frame are extended to a position where the wheels arespaced from the floor and pneumatic fluid is being supplied to saidinflatable members.

24. An extensible lift mechanism as defined in claim 23, in which saidinflatable members each include a flat rigid plate and an elastomericring attached to one surface of said plate and in which said lowersurface has guide means for receiving said plate to releasably supportsaid inflatable member on said surface.

25. An extensible lift mechanism as defined in claim 23, furtherincluding brake means for temporarily holding the floatingunit in afixed position on the support surface.

# l I i

1. In an extensible lift mechanism having a base frame and a loadsupporting frame connected for relative movement by a scissor linkage,first and second screws extending above said base frame at spacedlocations, said screws being supported for rotation at spaced parallelaxes on said base frame; reversible drive means cooperating with saidscrews for rotating said screws in opposite directions; first and secondcarriers respectively cooperating with said screws for axial movement inresponse to rotation of said screws; and first and second linkage meansbetween respective carriers and spaced points on said load supportingframe, said linkage means universally accommodating lateral movement ofsaid spaced points relative to said axes in response to differentialrotation of said screws to allow tilting of said load supporting framerelative to said axes.
 2. An extensible lift mechanism as defined inclaim 1, in which each linkage means includes links accommodatinglateral movement in two intersecting paths perpendicular to said axes ofsaid screws.
 3. An extensible lift mechanism as defined in claim 1, inwhich each linkage means includes a first link means pivoted on saidcarrier about a first pivot; a second link means pivoted on one of saidpoints on a second pivot parallel to said first pivot, said first andsecond pivots extending perpendicular to said fixed axes; and meansinterconnecting said first and second link means.
 4. An extensible liftmechanism as defined in claim 3, in which said means interconnectingsaid first and second link means includes a third link means pivoted atopposite ends about spaced parallel third and fourth pivots respectivelyon said first and second link means, said third and fourth pivotsextending perpendicular to said first and second pivots and said fixedaxes.
 5. An extensible lift mechanism as defined in claim 1, in whichsaid load supporting frame includes floor engaging elements, said floorengaging elements being dimensioned to raise said base frame above afloor upon movement of said carriers to an extreme position adjacentsaid base frame.
 6. An extensible lift mechanism as defined in claim 5,further including inflatable means defining recesses below said baseframe; and means for supplying pneumatic fluid to initially inflate saidinflatable means and flow into and from recesses between said inflatablemeans and the floor to produce a film of fluid to floatingly supportsaid base frame on the floor.
 7. An extensible lift mechanism as definedin claim 6, including brake means on said base frame for selectivelyholding said base frame in an adjusted position while said base frame isfloatingly supported on the floor.
 8. An extensible lift mechanism asdefined in claim 1, in which said frames are polygonal and said screwsare located at adjacent corners of one end of the said base frame.
 9. Anextensible lift mechanism as defined in claim 8, in which said scissorlinkage includes first and second pairs of arms pivotally interconnectedintermediately opposite ends, one end of each first arm pivoted, on oneend of said base frame adjacent said screws and one end of each secondarm pivoted about a fixed pivot on said load supporting frame adjacentsaid screws; guide track means on said base frame and said loadsupporting frame, movable pivots for opposite ends of said arms guidedin said track means, and in which all of said pivots for said armsaccommodate lateral movement of said frames relative to each other. 10.An extensible lift mechanism as defined in claim 1, further includingcontrol means coOperating with said reversible drive means forselectively rotating said screws in opposite directions to tilt saidload supporting frame relative to said base frame.
 11. An extensiblelift mechanism as defined in claim 10, in which said control meansincludes means for raising and lowering said load supporting framerelative to said base frame while maintaining a preset angle of tilt ofsaid load supporting frame relative to said base frame.
 12. Anextensible lift mechanism as defined in claim 11, in which said controlmeans includes means for automatically leveling said load supportingframe at opposite extreme limits of travel of said carriers on saidscrews.
 13. An extensible lift frame as defined in claim 1, furtherincluding control means for cooperating with said drive means forrotating said screws synchronously or independently.
 14. An extensiblelift frame as defined in claim 1, in which said control means includesmeans for simultaneously rotating said screws in the same direction orin opposite directions.
 15. An extensible lift mechanism as defined inclaim 1, in which said scissor linkage includes first and second pairsof arms pivotally interconnected intermediate opposite ends, each armhaving one end connected by a fixed pivot to one of said frames and anopposite end connected by a movable pivot to the other of said framesand in which said connections of said arms include spherical bearingsaccommodating tilting of said load supporting frame relative to saidbase frame.
 16. An extensible lift mechanism as defined in claim 1, inwhich said drive means includes a pneumatic motor for each of saidscrews.
 17. An extensible lift mechanism as defined in claim 16, furtherincluding releasable coupling means for interconnecting said motors tosynchronously drive both screws with both motors.
 18. In an extensiblelift mechanism having a horizontal base frame and a load supportingframe with scissor arms interposed between said frames; first and secondscrews extending above said base frame at spaced locations and supportedfor rotation about substantially vertical axes; drive means for rotatingsaid screws in opposite directions on said base frame; carrier meanscooperating with each screw; and linkage means connecting said carriermeans to said load supporting frame at spaced locations, said linkagemeans universally accommodating lateral movement of said load supportingframe relative to said vertical axes upon selective rotation of saidscrews for tilting said load supporting frame relative to said verticalaxes while minimizing lateral forces on said screws.
 19. An extensiblelift mechanism as defined in claim 18, in which said linkage meansincludes a plurality of links pivotally interconnected and pivoted tosaid carrier means and supporting frame to accommodate lateral movementof said supporting frame in two intersecting paths perpendicular to saidaxes.
 20. In an extensible lift mechanism including a base frame and aload supporting frame supported above said frame by scissor arms; acarrier; drive means for moving said carrier in opposite directionsalong a fixed axis extending above said base frame; and connection meansbetween said carrier and supporting frame, said connection meansincluding linkage means having opposite ends respectively pivoted aboutspaced parallel axes respectively defined on said carrier and saidsupporting frame, said spaced parallel axes extending perpendicular tosaid fixed axis and said linkage means universally accommodating lateralmovement of said supporting frame relative to said fixed axis.
 21. Anextensible lift mechanism as defined in claim 20, in which said linkagemeans includes a plurality of link means pivotally interconnected andpivoted about said spaced parallel axes to accommodate lateral movementof said supporting frame in plural directions perpendicular to saidfixed axis.
 22. An extensible lift mechanism as defined in claim 21, inwhich said linkage means includes a first link meAns pivoted on saidcarrier, a second link means pivoted on said supporting frame and thirdlink means pivoted at opposite ends on said first and second link meansrespectively.
 23. In an extensible lift mechanism including a base and aplatform; a scissor linkage interposed between said base and saidplatform; means for extending and retracting said platform above saidbase between collapsed and extended positions; a plurality of spacedwheels secured to said platform, said wheels extending below said baseto engage a support surface and support said lift mechanism when saidbase and platform are in a collapsed position; a plurality of inflatablemembers extending from a lower surface of said base; means forselectively supplying pneumatic fluid under pressure to said inflatablemembers; and means accommodating flow of fluid from said members toproduce a supporting film of fluid between the inflatable members andsaid support surface to support said lift mechanism on said supportingfilm of fluid when said base and frame are extended to a position wherethe wheels are spaced from the floor and pneumatic fluid is beingsupplied to said inflatable members.
 24. An extensible lift mechanism asdefined in claim 23, in which said inflatable members each include aflat rigid plate and an elastomeric ring attached to one surface of saidplate and in which said lower surface has guide means for receiving saidplate to releasably support said inflatable member on said surface. 25.An extensible lift mechanism as defined in claim 23, further includingbrake means for temporarily holding the floating unit in a fixedposition on the support surface.